test: fix test-debug-port-from-cmdline.js

Make this test less prone to race conditions by using synchronous
interprocess communication instead of a timer to determine when the
child process is ready to receive messages from its parent.

Also, remove a superfluous timer since the tests suite already makes
tests time out after a while.

Original-PR-URL: https://github.com/joyent/node/pull/9049


Reviewed-By: Timothy J Fontaine <tjfontaine@gmail.com>

PR-URL: https://github.com/iojs/io.js/pull/501


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Bert Belder <bertbelder@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>